Proper regulation of gene expression is critical to organismal function, yet many mutations that should significantly decrease expression are recessive. How do organisms deal with mutations that diminish gene expression? Are reduced levels of a gene's expression merely tolerated or are there mechanisms that rectify the expression itself? Such "regulatory compensation" has been documented in a few cases, but the generality of the phenomenon remains unclear. If regulatory compensation is common, it complicates the relationship between genotype and phenotype and has implications for understanding expression variation within and between species. The proposed research directly quantifies regulatory compensation on a genomic scale - over 72% of Saccharomyces cerevisiae's protein coding genes are interrogated. Gene deletion strains of S. cerevisiae are used to compare gene expression in cells of a homozygous wild-type genotype to cells that are heterozygous for the deletion allele. If compensation is absent, these heterozygotes should maintain only 50% of the gene product maintained by the wild-type homozyogte. Wild-type alleles will be tagged with a Green Fluorescent Protein (GFP) to allow allelic expression to be quantified in a high-throughput manner using flow cytometry. mRNA expression of the GFP-tagged allele will also be measured and compared to protein fluorescence to determine whether compensation is achieved in the abundance of transcript, protein, or both. Regulatory compensation is a natural but unproven explanation for dominance that might be more consistent with recent observations than the prevailing theory. This consistency will be tested by comparing the extent of compensation observed to estimates of dominance (derived from published data) for the wild-type allele relative to its deletion. Ultimately, the proposed experiments will (1) show how common regulatory compensation is on a genomic scale, (2) determine whether the mechanisms act transcriptionally, post-transcriptionally, or both, and (3) test whether patterns of regulatory compensation are consistent with patterns of dominance. Relevance: For a gene to be functional, an organism must have the ability to maintain the abundance of the gene's product. This study will measure yeast's ability to preserve the proper concentration of gene product in spite of no production from one of its two gene copies. Because the mechanisms for generating gene products in yeast are similar to those of complex organisms like humans, these findings will generate testable hypotheses about the consequences of regulatory mutations in other species.